1. Field of the Invention
This invention relates to the manufacture of integrated circuits, and specifically to a system for measuring and controlling the alignment of various layers in such circuits, as well as to a system for measuring critical dimensions of integrated circuits. The system permits alignment of masks and measurement of critical dimensions with accuracy greater than the resolution of the optical system employed in performing the alignment and measurements.
2. Description of the Prior Art
In the manufacture of integrated circuits, a semiconductor wafer, typically silicon, is subjected to a complex series of process operations to define active and passive components by doping regions within the wafer with impurities. During and after these operations, layers of electrically conductive and insulating material are deposited and defined on the wafer to interconnect the active and passive components into a desired integrated circuit. The processing of the wafer usually employs techniques in which masking layers of various materials, such as photoresist, are deposited across the upper surface of the wafer. Using photolithographic or other techniques, openings are defined in the photoresist or masking layers to allow selective introduction of P- and N-conductivity type dopants, such as boron, phosphorus, arsenic and antimony, through the surface of the silicon wafer. These doped regions provide components, such as transistor emitters, resistors, etc., of the integrated circuit. In state-of-the-art integrated circuit fabrication technology, many separate masks are employed to define the ultimate integrated circuit. For example, some bipolar circuit fabrication processes employ 13 different masks to selectively expose photoresist layers during different processes.
The economic revolution in electronics continues to be the result of the integrated circuit manufacturer's ability to place more and more components in a smaller and smaller area of the wafer. Because the cost of processing a single wafer is fixed, and substantially independent of the number of devices formed therein, decreasing the size of individual devices, increases the number of devices formed in a wafer, and results in lower cost per device.
As the individual components on an integrated circuit become progressively smaller, however, the importance of aligning each mask with the underlying wafer becomes greater. For example, if the minimum spacing between two electrically conductive lines on an integrated circuit is 5 microns, a 1-micron mask misalignment will not electrically short the lines to each other. On the other hand, a 1-micron misalignment on an integrated circuit having a minimum feature size of 1 micron will destroy the functionality of the circuit. Conductive lines will be shorted to each other, while transistor components will be so misplaced as to render the devices nonfunctional. Thus, as the integrated circuit industry's capability to place more components on a given size chip increases, the importance of properly aligning each overlying layer with the underlying wafer becomes greater.
One traditional approach to aligning or checking alignment of a layer with respect to the underlying structure, for example, a photoresist pattern deposited on the wafer, employs comb-shaped alignment patterns. A first comb-shaped pattern, for example, with teeth pointed north, is fabricated on the wafer in an early process operation. A complementary comb-shaped pattern with teeth facing south and with a slightly different spacing between the teeth is formed later, for example, in a photoresist pattern applied to the wafer. The second pattern is offset from the first pattern so that the tips of the teeth of the two patterns mesh. The slightly different spacings of the teeth allow only one pair of opposing teeth to be aligned with each other at a time. The position of the aligned pair in the comb pattern provides a sensitive measure of the alignment error between the two layers.
This vernier alignment pattern has proven satisfactory for many applications; however, distortion due to interference fringes in the optical images of the patterns make the line position difficult to determine. Furthermore, the area of comparison in the comb structure is a very small region where the teeth approach each other. Thus, the pattern may be employed effectively by automatic alignment measurement systems only if the imaging device by which the pattern is viewed has resolution as fine as the desired alignment measurement and has very low noise levels as well. Additionally, the inherent accuracy limit is determined by the digitizing grid used by the main circuit layout. It is desirable to overcome this limitation.
A further deficiency of present alignment patterns is that automatic measurement with the patterns requires complex software for identifying the patterns, recognizing the teeth, etc. Thus, automating the alignment of such patterns is difficult. Of course, aligning such patterns manually is undesirably labor intensive, and subject to operator interpretation. Such measurements are tedious and subjective, and the operator must key the results into a terminal to control a computer integrated manufacturing system.
Critical dimensions on a layer of an integrated circuit, as opposed to alignment of different layers, usually are measured by human operators. A test pattern fabricated on the circuit usually has a series of parallel bars, each having a width equal to the critical dimension, and each spaced apart from adjacent bars by the critical dimension. Overexposure, assuming positive photoresist, results in the bars being narrower than desired, consequently increasing the width of the spaces between them. Underexposure had the opposite effect, widening the bars and narrowing the spaces. Using a microscope, the human operator measures the critical dimension by comparing the bar/space ratio to assure that it is within tolerances. Of course, this approach requires operator intervention, and is susceptible to the same difficulties in interpretation as described above, that is, distortion due to interference fringes, distortion of the optical system used to examine the test pattern, and extreme difficulty in automating the measurement procedure.
In another approach, an automated system is employed whereby the bars and spaces are viewed with a microscope and television monitor. Using a gating network, one or more raster scans are selected for display on an oscilloscope to allow determination of bar/space dimensions. Unfortunately this approach suffers from the same disadvantages described above. Furthermore, it is difficult to determine precisely which part of the waveform corresponds to the edge of the bar or space.